This invention relates to communication of multisensor output signals in a noisy environment.
Improvement of bit error rate (BER) in a wireless system for digital communications is an elusive goal, in part because the approaches used for such improvement often depend upon the present state of the communication channel. For example, use of multiple antennas for signal reception improves the BER, and a method known as Maximal Ratio Combining (MRC), which uses all signals received at all antennas, is often used with a spatial diversity approach. However, the MRC may operate poorly in a multi-user co-channel interference environment, where an adaptive antenna array (AAA) approach often performs better. One problem that must be faced in an AAA approach is the moment-by-moment assignment of weights to the different antennas.
Some workers have used the MRC and AAA approaches together for signal combining. The AAA method is preferred where CCI (interference) is present in the channel, and the MRC approach is preferred when CCI is largely or wholly absent. In any MRC/AAA combination approach, the signal receiver must sense, or be provided with, information on the channel environment in order to choose an optimal approach for the particular channel condition.
What is needed is a method that senses and rapidly adapts to a changing channel environment and selects a wireless communication mechanism that is optimal based on the present condition of a channel. Preferably, the method should react rapidly and automatically to any substantial change in certain channel parameters. Preferably, the method should perform better than either the MRC approach or the AAA approach, used by itself. Preferably, the method should be flexible enough to allow use of any reasonable cost function and to allow some enhancements to be incorporated into the mechanism without requiring that the system be first shut down, reconfigured, and reactivated.
These needs are met by the invention, which uses a decomposition of a vector space defined by a group of sensor signals into a desired signal subspace and a noise subspace for each of two analytical approaches (Maximal Ratio Combining and Adaptive Antenna Array), with each approach being optimal in situations that do not overlap with optimal situations for the other approach. The two approaches are combined in a common procedure, but only one of these approaches is used in a given situation.
The apparatus includes an MRC apparatus and AAA apparatus that relies upon signal decomposition into a desired signal subspace V(s) and the remainder, a noise subspace V(n). The desired signal is preferably a selected reference signal, such as a CDMA signal or a frame synchronization unique word signal.